P0420 Catalyst System Efficiency Below Threshold?

1998 Cadillac 125,000 miles ( NOTE: I changed the Platinum sparkplugs at 100,000 miles and the car runs very smooth, no rough idling, or misfires, no other error codes). Also there is no funny smell. Someone told me that if I smell fuel or a rotten egg smell, then the cat is bad for sure.



Only thing about this car.... when I drive my car (2002 Cadillac), the instant reading on the MPG will go from 25 MPG to 70 almost instantly when I let off the gas. BUT on my wife's car ( the one with the error code) when I let off the gas it take about 3 or 4 seconds for it to go from 25 MPG to 70. And same thing when I press on the gas... a 3 second delay. Maybe this is normal but I just thought I would mention it in case it is relevant.



About 50 days ago this error code came on. This was about 5 days after my wife backed the car into a snow bank. After reading up on it, I was so sure my wife damaged the muffler or tail pipe. I did not see anything, so figured it could be a fluke. I used the scan tool to clear the error code and it was fine for about 50 days. Now the code came up again.So what I am thinking is that for the most part it is barely running within the limits, but once in a blue moon it fall out of specs and causes the %26quot;check engine soon%26quot; P0420 code to come up. So I am tempted to change the o2 sensors in front and back of the catalytic converter.

BTW I did clear the code again and it's been fine for 3 days. BUT I preparing myself for it to pop up again and will need to take care of it.



1) Is this hard to do ? I am just concerned that the heat had rusted the threads together and when I try to remove the Sensor, the treads will strip. Is this a problem ? I know I need to warm up the engine first to get the exhaust warm, but not too hot.

2) what about the converter ? Is this hard to change ? I am pretty sure the bolts will break, but i am wondering about the %26quot;hose clamp%26quot; in the back. How do I make sure that is sealed when I put the new one on ? Is there a gasket on back part ? is it funneled so I just put the cat pipe into the opening of the exhaust pipe then just clamp it with the %26quot;hose%26quot; type clamp ?



I do work on cars, but it is mostly water pumps, brakes, alternators, CV axles, etc...



Is there a way to check and see if the sensors are working ? I have a voltmeter.



Also, what about the O2 sensors that are on the exhaust manifold.. do I change those too, or should I change the 2 that are on each side of the catalytic converter ?



One more question... Is this the same as changing a sparkplug ? just unplug the wire, and replace sensor, then plug wire back in ? No need to reprogram the PCU or have it %26quot;learn%26quot; the new readings ?P0420 Catalyst System Efficiency Below Threshold?Alright there are a lot of questions here u hope i get them all. first. you need a new cat. sorry man no way around it. if it was the sensors then a sensor code would come up. unless so how all the o2 sensors went bad a the same time and if that was the case go buy a lottery ticket and u will be rich. your cat probably isn't too terrible if it takes a while for the light to come on, so feel free to try and get away with it as long as you can. the computer has to have three cases were the fault to happen in 3 differnt drive cycles for the light to come on.



O2 sensors are very easy to replace. u need a 22 mm wrench and that is about it. disconnect it and unscrew it. reverse to install. then ur done. no computers or anything. you can check o2 sensors with a volt meter but it wont tell ya much because the voltage is so low and you wont know what number it should be reading you need a scanner.



changing the cat shouldn't be to bad. i use the torch all the time with exhaust. get the metal around the bolt cherry red and remove the bolt. working it back and forth to loosen rust. if you don't have a torch or they just brake then drill out the bolt and use a nut and bolt to put it back together if it is nut and bolted to begin with then that's great brake it or just take it apart at for the other end the cat should slid over or in the exhaust tipe then you clamp it. put you hand over the tail pipe if it blows you hand off after a little then it is good. if it doesn't then plug it and see were it is leaking from.P0420 Catalyst System Efficiency Below Threshold?This will not be of any use for your present problem but then it may! Aprox 18 years ago in the UK bosch brought out platinum plugs. a REP wanted me to use them in my garage when weserviced a car,I tried a set in my 1961 land rover,BRILL better startng slight better mpg .So i started using them for S/ing.

Afew weeks later a call from a V/good customer saying car would not start(p/plugs used on last service) Igrabed some plugs and went out to him.changed the plugs (NON plat werenow put in ) the car started first time. this happend several times over the next few months.(different cars).

A bosch plug that had been removed was was then looked at i cut the earth prong off and the platinum electrode could then beseen .it had worn back so that the gap was 2 or 3 times the amount it should be!! no codes were on cars then so !!!!???